1. Field of the Invention
The present invention relates to an optical information recording medium, to an information recording method capable of recording and reproducing information by using a laser, and to a dye compound. In particular, the invention relates to a heat mode type optical information recording medium, to an information recording method suitable for recording information by using a short wavelength laser having a wavelength of 440 nm or less, and to a dye compound.
2. Description of the Related Art
An optical information recording medium (optical disc) capable of recording information only once by using a laser has been conventionally known. This disc is called a write-once type CD (the so-called CD-R). The disc typically has a structure in which a recording layer made of an organic dye, a light-reflective layer made of a metal such as gold and a protective layer made of a resin are laminated on a transparent disc substrate in that order. Information is recorded on the CD-R by radiating onto the CD-R near infrared ray laser beams, which usually have a wavelength of around 780 nm. Specifically, the laser-radiated portions of the recording layer absorb the light, so that the temperature of the portions rises locally. Consequently, the physical or chemical properties of the portions change (for example, generation of pits), and information is recorded as a result of the optical properties of the portions being changed.
Meanwhile, the information is read (reproduced) by radiating onto the recording layer laser beams having the same wavelength as those of the recording laser beams. Specifically, information is reproduced by detecting the difference between reflectivity of portions of the recording layer having a changed optical property (recorded portions) and reflectivity of portions of the recording layer having an unchanged optical property (non-recorded portions).
In recent years, optical information recording media having a higher recording density have been demanded. To meet such a demand, an optical disc called a recordable digital versatile disc (the so-called DVD-R) has appeared on the market (“Nikkei New Media”, special edition “DVD”, published in 1995). This DVD-R has a structure in which two discs each having a recording layer made of a dye, a light-reflective layer which is usually formed on the recording layer, and, if necessary, a protective layer on a transparent disc-shaped substrate having a guide groove (a pre-groove) which is tracked with radiated laser beams and whose width (i.e., 0.74 to 0.8 μm) is half or less of the width of grooves in CD-Rs, are bonded to each other with an adhesive so that the recording layer is disposed inside the DVD-R. Alternatively, the DVD-R has a structure in which the disc and a protective substrate having the same shape as the disc are bonded to each other with an adhesive so that the recording layer is disposed inside. The recording and reproducing of information in this DVD-R are performed by radiating onto the DVD-R visible laser beams, which usually have a wavelength of 630 to 680 nm. In the DVD-R, recordings of higher density can be attained than in the CD-R.
Recently, networks such as the Internet, and high-vision TV have been expanding rapidly. Moreover, the days of broadcasting by high definition television (HDTV) are also drawing near. Thus, demands are rising for large-capacity recording media capable of recording image information inexpensively and simply. Although the DVD-R is to some degree securing a position as a large-capacity recording medium, it cannot be said that the DVD-R has a level of recording capacity adequate to satisfy future demands. Thus, developments are being made in optical discs for which laser beams having a wavelength shorter than the wavelength of laser beams used for the DVD-R can be employed, and which discs accordingly have an improved recording density and a superior recording capacity.
For example, various recording and reproducing methods have been disclosed wherein an optical information recording medium having a recording layer containing an organic dye is used to record and reproduce information by radiating onto the medium from the recording layer side toward the light-reflective layer side laser beams having a wavelength of 530 nm or less (see, for example, Japanese Patent Applications Laid-Open (JP-A) Nos. 4-74690, 8-127174, 11-53758, 11-334204, 11-334205, 11-334206, 11-334207, 2000-43423, 2000-108513, 2000-113504, 2000-149320, and 2000-158818). Specifically, an information recording and reproducing method has been proposed for recording and reproducing information by radiating blue laser beams (wavelength: 430 or 488 nm) or bluish green laser beams (wavelength: 515 nm) onto an optical disc having a recording layer made of a dye such as a porphyrin compound, an azo dye, a metal azo dye, a quinophthalone dye, a trimethine cyanine dye, a dicyanovinylphenyl skeleton dye, or a coumalin compound.
As a result of research conducted by the inventor of the invention, however, it has been become evident that the above-mentioned optical discs are deficient in terms of recording properties such as practical sensitivity, reflectivity and degree of modulation and that further improvements are accordingly required. The optical discs are also deficient in terms of heat and humidity stability and in terms of light stability. Furthermore, there are also problems concerning the strength, or the film quality, of the recording film of the optical discs, and improvements in these properties are also necessary.
For the purpose of these improvements, JP-A No. 2002-172865 discloses an optical information recording medium having a recording layer including a 1,2,3-triazole compound and, in order to record information into the medium, a method of radiating onto this medium laser beams having an oscillation wavelength of 405 nm. In this method, however, the performance of optical information recording media stipulated as preferable examples has not yet proved satisfactory.
Accordingly, dye compounds and optical information recording media are required which are capable of recording and reproducing information by using laser beams having a shorter wavelength than those of laser beams used for CD-Rs or DVD-Rs, particularly laser beams which have a wavelength of 440 nm or less, and which also have excellent recording properties.
Dye compounds and optical information recording media are also required which are superior in terms of heat and humidity stability and in terms of light stability, and which are also satisfactory in terms of the strength and quality of the resultant recording layers.
Further, an information recording method is required which is capable of recording information at a high density, using an optical information recording medium having a recording layer which contains a dye compound having a high sensitivity to short wavelength laser beams.